Integrated circuits in planar electronic packages, otherwise known as flatpacks, produce significant quantities of heat which must be removed to maintain reliable operation. This requires a low temperature rise between the junction of the device and the heat sink. A major mode of heat transfer in many designs is conduction through the device package to a printed circuit board. Since the thermal resistance at the board/package interface is directly in series with heat flow, large temperature differences are possible between the board and the flatpack. The need, therefore, is to minimize thermal resistance at the component-board interface, thus lowering the component temperature by permitting free transfer of heat.
Present methods of providing a thermal path include the use of double-sided tapes, silicone greases, epoxies, silicone rubbers, and other thermal compounds.
The double sided tapes are not conductive and thus, to permit heat transfer, the tapes must be placed so that they do not fill the entire space between the device and the board. Silicone grease, as well as many other compounds, cannot withstand the necessary chemical cleaning. Triclorethane, a common PC board cleaner, dissolves most of the non-curing materials tested. In addition, the accidental exposure of a solder pad to silicone has adverse effects on solderability.
Epoxies can provide a good thermal path, but installation is permanent. The component cannot be removed without damaging the board, the device, or both. Silicone rubber provides a rather poor path due to low thermal conductivity. Repair, while not as difficult as epoxy is not an easy procedure.